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(jpg, 181 KB)The Aletschglacier in Switzerland is a typical valley glacier and the largest one in the Alps. Its volume loss since the middle of the 19th century is well visible from the trimlines to the right of the image. (Image:
Frank Paul)

(jpg, 145 KB)Local glaciers and ice caps in eastern Greenland in the foreground and the ice sheet with its outlet glaciers (upper right) in the background. (Image:
Frank Paul)

News release, May 16, 2013

Sea level: one third of its rise comes from melting mountain glaciers

About 99% of the world’s land ice is stored in the huge ice sheets of Antarctica and Greenland, while only 1% is contained in glaciers. However, the meltwater of glaciers contributed almost as much to the rise in sea level in the period 2003 to 2009 as the two ice sheets: about one third. This is one of the results of an international study with the involvement of geographers from the University of Zurich.

How much all glaciers contribute to global sea-level rise has never been calculated before with this accuracy. An international group of researchers involving two geographers from the University of Zurich has confirmed that melting of glaciers caused about one third of the observed sea-level rise, while the ice sheets and thermal expansion of sea water account for one third each. So far, estimates on the contribution of glaciers have differed substantially. Now 16 scientists from nine countries have compared the data from traditional measurements on the ground with satellite data from the NASA missions ICESat (Ice, Cloud and land Elevation Satellite) and GRACE (Gravity Recovery and Climate Experiment).

Combined with a glacier inventory that is available globally for the first time, the researchers were able to determine the glacier mass changes all over the world much more accurately than before. «The extrapolations of local field measurements to large regions and entire mountain ranges traditionally applied sometimes overestimated the ice loss», describes UZH geographer Frank Paul the findings from the satellite measurements. And his fellow colleague Tobias Bolch adds: «We are well aware of the weaknesses of the individual satellite methods. However, in highly glacierized regions the results obtained using the two different methods agree well. With the mix of methods that have now been tested and applied, we have come a major step closer to determining glacier mass loss with higher precision.»

Earlier estimates should be corrected

The results show that almost all glacier regions lost mass in the years 2003 to 2009, most of all in Arctic Canada, Alaska, coastal Greenland, the southern Andes and in the Himalayas. By contrast, the glaciers in Antarctica – smaller ice masses that are not connected to the ice sheet – made scarcely any contribution to sea-level rise during this period. This finding deviates significantly from previous estimates, saying that the Antarctic glaciers caused around 30% of the global ice loss in the period from 1961 to 2004. «However, neither the periods nor the data basis are directly comparable here», adds Bolch, «so we shouldn’t make any premature conclusions in this respect.»

The results published in «Science» have important consequences for past studies: Bolch and Paul conclude by recommending that «Earlier global estimates on the contribution of glaciers to sea-level rise should be revised again».

Background

While GRACE determines the changes in the Earth’s gravity field and thus can only calculate mean values over large regions, hundreds of kilometers across, that are heavily glacierized, ICESat is fitted with lasers that record each 170 meter the distance from the Earth’s surface along predetermined paths with a spatial resolution of 70 meters.

The UZH geographers Tobias Bolch and Frank Paul contributed important basic data for the study: digital glacier outlines for the global glacier inventory from different regions in the world for which no precise data were previously available. For example for Alaska, Baffin Island, Greenland, the Alps, high-mountain Asia including the Himalayas as well as own calculations on the mass loss in Greenland and in high-mountain Asia.